This invention relates to high-temperature, secondary electrochemical cells and batteries of such cells that can be employed as power sources for electrical automobiles, hybrid electric vehicles or for the storage of energy generated during intervals of off-peak power consumption. It is particularly applicable to electrochemical cells that employ metal sulfides as positive electrode reactants and alkali metals as negative electrode reactants.
A substantial amount of work has been done on the development of these types of electrochemical cells and their electrodes. The various type cells showing promise have employed lithium, lithium-aluminum alloy or sodium as the reactant or active material within the negative electrode. In the positive electrode, the chalcogens, particularly sulfur and sulfur compounds, have been used. Electrolytes of molten salt, generally containing the ions of the negative reactant, provide ionic conduction between the electrodes. Examples of these secondary, high-temperature cells are disclosed in U.S. Pat. No. 3,887,396 to Walsh et al. entitled "Modular Electrochemical Cell", June 3, 1975; U.S. Pat. No. 3,884,715 to Cairns et al. entitled "Secondary Electrochemical Power Producing Cells Having Mixed Cathode Compositions, May 20, 1975; U.S. Pat. No. 3,827,910 to Cairns et al. entitled "Homogeneous Cathode Mixtures for Secondary Electrochemical Power-Producing Cells", Aug. 6, 1974; U.S. Pat. No. 3,716,409 to Cairns et al., entitled "Cathodes for Secondary Electrochemical Power-Producing Cells", Feb. 13, 1973; and U.S. Pat. No. 3,488,221 to Hiroshi Shimotake et al., Jan. 6, 1970. A number of other patent applications relating to these type cells include ERDA Case No. S-44,319, Ser. No. 434,459 now U.S. Pat. No. 3,907,589, issued Sept. 23, 1975, by Gay and Martino, entitled "Cathode for a Secondary Electrochemical Cell"; and Ser. No. 510,840, filed Sept. 30, 1974 by Yao and Walsh, entitled "Electrochemical Cell Assembled in Discharged State" now U.S. Pat. No. 3,947,291 Mar. 30, 1976. Each of these patents and patent applications are assigned to the assignee of the present application.
The iron sulfides FeS.sub.2 and FeS have been found to be particularly attractive sulfur compounds for use as positive electrode reactants. These materials are readily available and are much more easily contained within the cell than elemental sulfur. Although FeS.sub.2 has a lower equivalent weight and generally performs better within the positive electrodes than FeS, it reacts with and degrades iron components within the cell to form FeS. Consequently, inert materials such as molybdenum, tungsten or carbon are required for use in current collectors and electrical terminals in contact with FeS.sub.2 cathode compositions. Unfortunately, these materials are difficult to weld and otherwise fabricate into useful structures. Molybdenum in particular has been extensively used as a current collector with FeS.sub.2 positive electrodes. This current collector material contributes substantially to the weight and cost of the cell.
Various other problems have been encountered in the development of iron disulfide positive electrodes. A substantial volume increase occurs as this active positive electrode material reacts with lithium ions within the electrolyte to form lithium sulfide reaction product. The expansion may rupture or deform the electrode containment to reduce cell performance. In addition, various intermediate phases are formed within the positive electrode, for instance solid phases of Li.sub.2 FeS.sub.2, Li.sub.4 Fe.sub.2 S.sub.5, etc. Certain of these phases may interfere with the efficient utilization of the electrode active materials.